Well for you it looks like the most direct way would lead you through Alberta to the beginning of the Alaska Hwy in Dawson Creek BC. If you want a small detour you can come and visit us 1hr west of Dawson Creek before you head up the Alaska Hwy.
Fuel is getting pretty pricey up here, Diesel costs more now then Gas, Diesel around C$ 1.28 per lt and of course for summer vacation time this will for sure go up more. They predict up to 1.5 / lt, as further North you get as more expensive it will be.
If you come up from Grande Prairie fill your tanks at the boarder crossing from Alberta to BC. BC is most of the time more expensive then Alberta.
Dawson Creek is cheaper then Fort St.John / Fort St.John is cheaper then Fort Nelson. You'll have a long stretch between Fort St.John and Fort Nelson (400 + km's) that will have the occasional small gas station only open during the day (or might be closed due to lack of business) at overpriced rate. Same between Fort Nelson and Watson Lake. The rule up here is, unless you have a huge tank that will get you 400 + km with whats left in it fill up if you got a chance to do so. Braking down, running out of fuel will be expensive because the next service station could be a few hundred km's away. Parts are kind of hard to get and will take a few days to get to you if your lucky.
I'm planning a trip up there myself either with Skoolie or motorbike this summer so I would be able to give you a little more up to date report later on. (last time i went up was last summer, only to Withehorse Yukon, and many of the smaller gas stations that used to be in business about 5 years ago are now abandoned)